1. Field of the Invention
The present invention relates to an apparatus and method for passive geophysical prospecting. More particularly, the present invention relates to detecting at the Earth's surface in a non-invasive manner subsurface discontinuities associated with extremely low frequency electromagnetic fields.
2. Description of Related Art
The art is replete with various passive methods and associated apparatusfor passive geophysical prospecting. There is great motivation in discovering a reliable method of this type which is simple and therefore relatively inexpensive when compared to actually drilling or performing non-passive geophysical prospecting, e.g., seismic measurements and the associated and expensive computer manipulation of resulting data, in a known area having discontinuous strata or in an unknown area, i.e., wildcat territory, or to determine if zones of a producing formation were missed or just beyond the terminus of an existing well.
Several passive methods utilize an antenna to pick up naturally occurring frequencies emanating from the Earth's surface. Typically, the received signal is amplified, filtered and detected. See for example U.S. Pat. No. 5,148,110 to Helms which detects a time varying signal emanating from the Earth's surface. U.S. Pat. No. 4,686,475 to Kober et al. detects the vertical electric field component of telluric currents using a special antenna and a tunable RC filter with detection be performed in an audio manner using the ears of an operator. This method is subjective to the operator and therefore suffers in reliability and consistency.
There is much noise associated with or interfering with these low frequency signals.
This one reason why low pass and high pass filtering is employed after the initial amplification of the signal. However, because of the initial low frequencies of the signal, it is difficult to discern the valuable information it carries and conditioning is preferred. Those skilled in the art have been trying for a long time without great success to find the proper way of conditioning the received signal to discern this valuable information in a consistent and reliable manner. It is for this reason that such passive techniques have not been accepted by the hydrocarbon prospecting community and relegated to the level of "divining rods".
One problem is that simple amplification is typically not sufficient to allow the filters to operate effectively. For this reason a frequency generated by an oscillator has been added to the signal in certain methods using a mixer to add amplitude and furnish a reference frequency for filtering the received signal. See for example, U.S. Pat. No. 3,087,111 to Lehan et al.(amplifies signal and then adds the oscillator frequency), U.S. Pat. No. 3,197,704 to Simon et al. and U.S. Pat. No. 4,198,596 to Waeselynck et al. (amplifies, filters, adds oscillator frequency and then low pass filters). For the most part, the oscillator controls the center pass frequency of the filters being used. However, the quality of the received signal is not enhanced and the problems of reliability and consistency remain.